A stampable sheet is a sheet form article, in which a thermoplastic resin is impregnated in reinforcement fibers. There are various stampable sheets depending upon production processes and forms of reinforcement fibers. As production processes of the stampable sheet, a lamination method and dispersing method have been known.
The lamination method is a method in which a fiber mat first is prepared from reinforcement fibers, the fiber mat and a thermoplastic resin sheet or molten thermoplastic resin sheet are stacked, and then, the thermoplastic resin is impregnated into the fiber mat by applying heat and pressure (U.S. Pat. Nos. 3,644,909, 3,713,962 and 3,850,723).
On the other hand, the dispersing method is a method for producing the stampable sheet in which after mixing and dispersing reinforcement fibers and thermoplastic resin, the mixture is shaped into a sheet form, and heat and pressure are applied to the sheet form mixture. This method is divided into a wet method to perform mixing and dispersing in water (Japanese Examined Patent Publication (Kokoku) No. Heisei 2-48423, Japanese Examined Patent Publication No. Heisei 3-4678 and Japanese Examined Patent Publication No. Heisei 4-40372) and a dry method to perform mixing and dispersing within a special mixer without using water (Japanese Examined Patent Publication No. Heisei 3-55312, Japanese Examined Patent Publication No. Heisei 2-42058 and Japanese Unexamined Patent Publication (Kokai) No. Heisei 4-173311). In the wet method, reinforcement short fibers and thermoplastic resin powder are mixed in water, made into a sheet by a paper machine by adding a flocculating agent or a binder, and water is removed to dry and to form a sheet. A plurality of sheets are stacked as required, and heat and pressure are applied to obtain the stampable sheet. On the other hand, in the dry method, reinforcement fibers and thermo-plastic resin powder are mixed within a special mixing chamber, formed into sheet form on a belt, and heat and pressure are applied to form the stampable sheet.
The stampable sheet formed by lamination method has high impact strength since the reinforcement fibers are continuous fibers. However, it is difficult to cause local orientation due to mechanical entangling of the fibers. On the other hand, since the reinforcement fibers cannot be sufficiently filled into a rib or a boss, the design strength cannot be achieved. Therefore, freedom in designing of products is limited.
On the other hand, the stampable sheet produced by dispersing method has shorter fibers in comparison with the lamination method. This easily causes fluctuations of strength due to local orientation of the reinforcement fibers. Such sheet also has low impact strength. On the other hand, since the reinforcement fibers can be filled even into a rib or boss, designing of the products is easy.
Therefore, there have been several proposals for obtaining the features of short fiber reinforcement in a stampable sheet of long fiber reinforcement typically obtained by the lamination method.
For example, examples of attempts for improvement by impregnating a resin containing short fibers in a long fiber mat have been disclosed in Japanese Unexamined Patent Publication No. Showa 48-80172, Japanese Unexamined Patent Publication No. Showa 54-21476, Japanese Unexamined Patent Publication No. Showa 55-161618, Japanese Unexamined Patent Publication No. Showa 57-100029 and Japanese Examined Patent Publication No. Heisei 4-65854. However, in such methods, as pointed out in Japanese Unexamined Patent Publication No. Showa 64-4320, most of the short fibers are filtered by the long fiber mat to penetrate almost only resin into the mat. Then, most of the short fibers form a layer in the vicinity of the surface of the long fiber mat. When stamp forming is performed with such stampable sheet, the short fibers may occupy most of the proportion of the reinforcement fibers to be filled, in a rib, boss and end portion of a formed product. Conversely, in a charge portion of the original sheet, the long fibers may occupy most of the proportion. As a result, differences of proportions of the long fibers and the short fibers can be caused at various portions of the product. This causes differences in impact strength and fatigue characteristics, or, anisotropy may be caused due to local orientation of the short fibers. Furthermore, distortions also may be caused.
While not directed to a stampable sheet, as a sheet form material of a fiber reinforced, thermoplastic resin, which can be formed by pressure forming, and in which the long fibers and the short fibers are combined, a long fiber mat and a short fiber mat are stacked and thermoplastic resin is impregnated into the stacked mats, as disclosed in Japanese Unexamined Patent Publication No. Showa 58-98241. In this publication, one long fiber mat and short fiber mat are stacked and the thermo-plastic resin in impregnated thereinto. From this, it can be considered that a stampable sheet, in which continuous fibers and the short fibers are uniformly distributed, easily can be obtained by stacking a plurality of thin continuous fiber mats and thin short fiber mats and formed into a mat by needle punching, and impregnating thermoplastic resin into the needle punched mat. However, when this is practiced, as shown in FIG. 7, for example, production units (A) of the continuous fiber mat and production units (B) of the short fiber mat have to be arranged alternately along the transporting direction of a conveyer C. This requires a huge capital investment.
Then, upon needle punching the continuous fiber mat, when punching density is increased or the penetration depth of the needles is made deeper, cutting of the fibers is increased. Thus, a mat in which the short fibers are distributed in the continuous fiber can be obtained. By impregnating the thermo-plastic resin in this mat, a stampable sheet can be obtained.
However, such stampable sheet has the following drawback. Namely, when the punching density is increased, entangling of the fibers can become stronger to make the mat difficult to loosen upon forming. This requires a higher forming pressure. On the other hand, when the penetration depth of the needles is made deeper, the number of fibers oriented in the thickness direction is increased, thus to increase the loft (nominal thickness) of the mat. As a result, upon heating, a blank (hot blank) is expanded significantly to cause a tempera- ture difference between an inner layer thereof and a surface layer thereof. If the heating period is expanded for eliminat- ing such temperature difference, the surface layer can be overheated to degrade appearance. Furthermore, heating aging can be promoted to degrade physical properties.
Therefore, the object of the present invention is to provide a glass fiber mat for a stampable sheet, in which long fibers and short fibers are not separated and are distributed uniformly even in a forming process, and which has the effective characteristics of the respective fibers, a production process thereof, a stampable sheet employing such glass fiber mat, a production process thereof, and a production system therefor.